Nineteen-year-old Kasumi is the eldest daughter of the Tendo family. She stated that she finds younger men boring when she and her sisters learned of Ranma's pending arrival. After Nabiki discovered that he was catatonic and walked away in disgust Akane mistook her sister's angered face for something that Ranma must have done and attacked him in a fit of rage. When Kasumi called her out for this she just left feeling safe with the knowledge that he wasn't on her level.
Kasumi then went to check up on Ranma when fhe didn’t find any injury she tried to get him to the living room only he proved too heavy. After Ranma got up when she attempted to pull him by his arm and started following her when she asked him to. After Genma explained his version of what happened Kasumi seeing Ranma remaining Ranma remained oblivious to the negative feelings directed his way decided to lead him to his room. Nabiki helped as well as she wanted to get away from her younger sister, who was questioning Genma about his knowledge in martial arts with an eager smile on her face.
Months passed in a relatively monotonous way. Akane trained with Genma and Soun, who regained his passion for the Art with the arrival of his friend and his daughter's dedication. Akane's skill in the Saotome branch of the Anything Goes was very limited due to her temper and tendency to use strength instead of speed, but she advanced pretty well on the Tendo branch. Soun and Genma sparred everyday and congratulated themselves for starting a class in the afternoons. Soun, Akane and Genma ignored Ranma's existence as best as they could.
Kasumi spent the entire day with Ranma close behind her. She hurt to see him like that, but was happy for the company he provided and the help he gave around the house. Even after her father and sisters told her Ranma didn't listen to her she insisted on talking to the young man, who she liked to have around. He would accompany her grocery shopping, polish the dojo floor, dry the dishes as she washed after the meals. In general, he gave her lonely life company. She would always smile pleasantly at him, not because she felt forced to do it, but because she was genuinely grateful to him. Her life was one of loneliness since her mother died, but now she had someone that not only depended on her, but also helped with her chores. She didn't give him things to do to ease her own workload, though. She did it in the hope that he someday got out of the state he was caught in. Maybe doing these little things was helping him.
Unfortunately one such shopping trip Kasumi and Ranma were attacked by the Black Dragon’s. Fortunately though this attack broke Ranma out of his catatonic state and he dealt with all the attackers before they could do anything to her. After taking Kasumi home he was going to find the rest of the gang to deal with them only for her to ask him not to. Both because she didn’t want anymore violence as well as not wanting to be alone.
Ranma and Kasumi were in the dojo cleaning weapons when Akane came in and once again attacked him in a fit of rage. When Kasumi tried to stop her she blindly lashed out startling Kasumi and making her fall to the ground. When the rest of the family arrived in the dojo only to see Ranma glaring down at Akane for attacking Kasumi they mistook the scene as Akane trying to protect Kasumi from a violent Ranma and Akane in her delusional state went with it.
Kasumi could only watch incredulously what was happening. She tried to yell at them to stop, to explain what had really happened. She tried so hard. Yet she couldn't. The violence only brought to the surface the Black Dragon attack and she froze, staring at the scene as if she wasn't there at all. The next day after calming down and realizing that given Ranma’s strength her sister wouldn’t be here if he had attacked her Nabiki confronted Kasumi and got her to admit what really happened.
Since Kasumi didn’t want there to fights within the family Nabiki agreed not to tell her father what happened. She later found him in the park and after talking to him Nabiki began to wonder what was wrong with Kasumi and Ranma as they were both trying to make sure the other wasn't hurt and ended up blaming themselves for others' doing. Giving up on it she took him back to the dojo to get his pack. Just as Nabiki predicted Kasumi was the only one in the house when they arrived so she left the two alone to make sure they weren't caught unaware by any of the people in the dojo. In the awkwardness of the moment Kasumi brought up how Ranma was kicked out of the dojo and oblivious to her interest Ranma explained how Hinako saved him from falling in again and was kind enough to offer him a place to stay. Taking Ranma to his pack they both apologized for what happened. Before he left Ranma gave Kasumi a ring that could be used to contact him if she is ever in trouble.
Soun eventually discovered that Nabiki was helping Ranma and was convinced that he was somehow forcing her. By this point Nabiki was getting tired of Kasumi’s passive nature as all it would do is end up getting Akane and their father killed is she didn’t tell the truth. While she didn’t like calling Kasumi out like that after hearing what Ranma did to some Black Dragon members she felt it was for the best. Their fight was interrupted by Shampoo tracking Genma down.
The same day that Aeris was enrolled in Furinkan high it was attacked by Sephiroth and Nabiki was nearly killed. Luckily Ranma was able to stop him and heal her wounds. Finding Kasumi drinking tea with Shampoo she explained what happened when they were interrupted by Soun and Genma. By this point Nabiki had lost almost all respect for her father so when Soun demanded to know the wear abouts of Ranma she just snapped to do what. Even when he slap her and demanded that she respect him in his house she reminded him that the house was there because of her. Kasumi seeing this realized that it was finally time to come cleam and had Shampoo take Nabiki to her room.